Dwarf Archives: photometry and references for M, L and T dwarfs
Sandy Leggett's L and T dwarfs page: NIR and MIR photometry and spectra
SpexPrism libraries: low-res spectra for M, L and T dwarfs and spectral standards
WFCAM Science Archive search tools for UKIDSS, etc.
Widefield Survey Explorer: databases for WISE
SDSS: the Sloan Digital Sky Survey DR7 Finding Chart
Astro-ph: new papers on Arxiv
ADS abstracts: the Astrophysics Data Service search page
SIMBAD: SIMBAD Astronomical Database
VIZIER: VIZIER catalogue service
HEASARC: online co-ordinate converter
Systemic: exoplanets blog (not directly my field, but interesting reading nonetheless)
Bad Astronomy: the Bad Astronomy blog
Centauri Dreams: speculative blog about the science of interstellar travel
Pharynglua: PZ Myers' biology blog
Dark Faculae: my science/current affairs blog, currently on semi-hiatus.